Another way to seal a negative is by using plaster bandages in place of duct tape. Plaster bandages are loose, net-like strips of fabric that are impregnated with plaster dust. Much like the regular plaster that you mix; when exposed to water, the plaster bandages will harden into a solid. Placing wet bandages over the seam in a negative will make a very strong water tight seal. If used correctly plaster bandages can create very durable, leak proof seams even on oddly shaped casts. However, plaster bandages are very expensive and generally not used for our projects at GBC.
Plaster bandages come in long rolls. To begin you will need to unravel some bandage...
... And tear off a strip of an appropriate length. This depends largely on the shape of the seam that you are working with.
Fold the strip in half and submerge it in water. This is best done by holding each end and allowing the strip to go slack. Start dunking at the middle and progress to the edges until the entire strip is wet. Holding onto the ends ensures that you won't lose them when the bandage gets floppy and sticks to itself.
Then lay the bandage over the seam, patting and pushing it down so that the mesh sticks together and there are no holes. Depending on the cast it may be necessary to tack the negative shut with a clamp or piece of duct tape so it won't spring open before the plaster bandage dries (it is usually hard enough to hold a negative shut after a few minutes). Once the entire seam is sealed you are free to fill your cast.
Some casts, like the transfemoral socket below, do not have even brims.
A transfemoral negative viewed from the medial aspect.
In this case, we want our cast to extend to the top of the back wall (this is in fact the lateral wall of the cast; we learn about transfemoral sockets in second year). But the other walls of the cast do not extend this high up.
To solve this problem we can build up the lower brims of the negative cast with layers of duct tape. Adding them one on top of the other, distally to proximally, until we've evened out the height of the brims. About half an inch of overlap on the duct tape strips should create a plaster proof seal.